Bring back the timestamp task/batch app for 2.0.x (#192)

- Remove unused timestamp task apps `docker/assembly.xml`
- Resurrect the timestamp task/batch 2.0.x app
- Add the resurrected apps to CI which publishes them to `/milestone`

timestamp-task-2.0.x/src/main/java/io/spring/timestamp/TaskApplication.java

@@ -0,0 +1,68 @@
+/*
+ * Copyright 2021  the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.spring.timestamp;
+
+
+import java.text.DateFormat;
+import java.text.SimpleDateFormat;
+import java.util.Date;
+
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.CommandLineRunner;
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+import org.springframework.boot.context.properties.EnableConfigurationProperties;
+import org.springframework.cloud.task.configuration.EnableTask;
+import org.springframework.context.annotation.Bean;
+
+/**
+ * Spring Boot Application that has tasks enabled.
+ */
+@EnableTask
+@SpringBootApplication
+@EnableConfigurationProperties({TimestampTaskProperties.class})
+public class TaskApplication {
+
+	private static final Log logger = LogFactory.getLog(TaskApplication.class);
+
+	public static void main(String[] args) {
+		SpringApplication.run(TaskApplication.class, args);
+	}
+
+	@Bean
+	public TimestampTask timeStampTask() {
+		return new TimestampTask();
+	}
+
+	/**
+	 * A commandline runner that prints a timestamp.
+	 */
+	public static class TimestampTask implements CommandLineRunner {
+
+		@Autowired
+		private TimestampTaskProperties config;
+
+		@Override
+		public void run(String... strings) throws Exception {
+			DateFormat dateFormat = new SimpleDateFormat(this.config.getFormat());
+			logger.info(dateFormat.format(new Date()));
+		}
+	}
+}